salt in which all the qualities of sweetness or saltness are preserved. These molecules, however, although the elements of substances, are not the ultimate elements of matter, but on the contrary are themselves aggregates—often very complex aggregates—of still smaller units which are considered to be the elemental atoms. Of such atoms we must admit as many different kinds as we have distinguished chemical elements, and the atoms are for the present the ultimate limit of our analysis of matter. These atoms are now the ideal chemical elements. Starting from the atoms, the orders of combination are, first, the union of the atoms to form the molecules which are the nuclei of definite substances, then the aggregation of these molecules to form material masses.
Obviously we may conceive of the union of either similar or of dissimilar atoms; and while the union of unlike atoms results in the production of molecules of compound substances, the union of like atoms (all of oxygen or all of hydrogen, for example) yields molecules of elementary substances. So far as the primary structure is concerned, there is no distinction between an elementary substance like oxygen gas and a compound substance like water. In each case the material is an aggregate of similar molecules, and owes its physical qualities to the external relations of its peculiar units; but, while the molecules of oxygen gas are each composed of two atoms of oxygen, the molecules of water consist each of two atoms of hydrogen and one of oxygen. .
In admitting the possibility of the union of similar atoms to form the molecules of elementary substances, the new philosophy of chemistry differs most markedly from the old. The system of Lavoisier was based on a conception of dualism originally suggested by sexual relations; and the term elective affinity, which was so constantly used to explain chemical changes, was a phase of this conception. The elements of two kinds paired together to form acids or bases, and the acids and bases paired to form salts, and chemical changes were the consequence of the superior affinity of another acid or another base for the temporary mate of a fellow-companion. At the time of Lavoisier, the grosser features of these dualistic conceptions, which so disgust us in the earlier writers on chemistry, had disappeared; and, still later, Berzelius attempted to place the system on a scientific basis by referring the dualism to electrical relations. But there was an entire continuity of thought from first to last, and in this was involved the prevailing idea that strength of chemical union depended on opposition of qualities. But this idea, which I have no doubt many scholars who studied chemistry under the old system still retain, was an entire misconception.
One of the strongest combining forces known to chemistry is that which holds together the dissimilar atoms of oxygen and
